The present invention relates to processes for producing esters from the reaction of an organic acid and an alcohol.
Esters are organic chemicals of significant industrial importance, for example for use as solvents and as reagents. One way to form esters is by reacting an organic acid with an alcohol to form an ester and water, as shown in reaction (1)
R1xe2x80x94COOH+R2xe2x80x94CH2OH⇄R1xe2x80x94COOxe2x80x94CH2R2+H2Oxe2x80x83xe2x80x83Reaction (1)
organic acid+alcohol⇄ester+water
Many esterification processes or systems focus on removal of water to drive the yield or conversion. Removal of water biases the equilibrium towards the products shown on the right hand side of Equation (1). This approach to esterification has been applied to a wide range of organic acids and alcohols.
For example, long chain alcohols that form heterogencous azeotropes with water can be used for water removal in the reaction""s overhead vapors. Water removal can be easily carried out by using a higher alcohol that is sufficiently high boiling and slightly soluble in water. In addition, excess alcohol can be used to drive reaction.
Alternatively, an added azeotroping agent may be used to remove water in the case of esterification using lower alcohols such as ethanol and methanol. One example of a suitable azeotroping agent is benzene.
However, this approach to water removal to drive the reaction has met with less success with systems wherein one or both of the reagent components tend to form dimers, oligomers, polymers or side reaction products when they are dehydrated. Esterification of lactic acid is one such example. In these cases, progress has been restricted by the problem that while water removal is necessary to drive the equilibrium of reaction (1), this water removal at the same time also produces unwanted dehydration side reactions such as formation of various dimers and oligomers or lactic acid. These lead to yield loss.
The problem of oligomer formation has in the past been partially overcome by addition of significant excess alcohol, which tends to suppress the reactions that lead to formation of dimers and oligomers. However, this approach has not been entirely successful and also it has lead to higher costs for recovery of the product due to high levels of excess alcohol that must be removed.
Another approach that has been used is a single reactor wherein a batch of lactic acid or other acid is dehydrated, or is present initially at high concentration and is heated such that when the alcohol is introduced into the vessel, then ester, excess alcohol, and water formed in the reaction are flashed out of the reaction vessel.
One example of the esterification of lactic acid is given by Gabriel et al (U.S. Pat. No. 1,668,806) who prepared 1-butyl lactate by dehydrating 70% lactic acid with excess 1-butanol at 117xc2x0 C., followed by addition of HCI catalyst, followed by refluxing and esterification with addition of excess 1-butanol and drawing a 1-butanol water azeotrope overhead. The process involved dehydration of the system and removal of water prior to the esterification step.
Bannister (U.S. Pat. No. 2,029,694) describes a method for producing esters that have boiling points of at least 120xc2x0 C. The lactic acid and acidic catalyst are charged to a reactor and heated to the boiling point of the ester or not less than 20xc2x0 C. below this temperature. The alcohol is introduced into the reactor below the surface of the hot partially dehydrated acid. The ester, water of reaction, and excess alcohol are taken off overhead. For example, methyl lactate is formed at temperatures from 130 to 140xc2x0 C. by introducing methanol into partially dehydrated lactic acid. The overhead distillate is 8-10% water, 42-42% methanol, and 50% methyl lactate, by weight. For every 4.8 moles of methyl lactate produced in the system a total of 17.9 moles of methanol is fed to the system. Most or all of the water taken overhead (5.0 moles) is produced by the esterification reaction. The effective feed water level is 0.2 moles. This means that the feed streams are essentially water free.
Weisberg, Stimpson, and Miller (U.S. Pat. No. 2,465,772) mix substantially water-free lactic acid with 3 to 20 parts by weight of aliphatic alcohol of 1 to 3 carbon atoms, reacting the mixture below the boiling point and then flashing the mixture at a higher temperature. For example, for the case of formation of methyl-lactate, the mole ratio of methanol to lactic acid is at a minimum 8.5:1. It may be as high as 56 moles of methanol per mole of lactic acid.
Filachione and Fisher (Industrial Engineering and Chemistry, Volume 38, page 228, 1946) present another example of such technology. Their scheme involves bubbling excess hot alcohol, such as methanol vapor, through a hot partially dehydrated lactic acid solution at a temperature above the alcohol boiling point, whereby the lactate ester produced is removed with the alcohol vapors and any water produced from the reaction. Approximately 9 moles of methanol are required per mole of lactic acid from an 82% solution. Dramatically larger quantities of methanol are required for more dilute lactic acid feed solutions.
These methods described above typically require both excess alcohol and also dehydrated lactic acid. They are not energy efficient and also they require equipment that is large and expensive. In the cases where dehydrated lactic acid is used, the reaction temperature is typically close to that of the boiling point of the ester. In each case the reaction is conducted at the same temperature as the boiling or mass transfer.
An alternate approach is to attempt the reaction without first dehydrating the lactic acid. The following two references illustrate previous attempts that have been made to effectively utilize such an approach.
Wenker (U.S. Pat. No. 2,334,524) describes a process wherein both an esterification reactor and an adjacent hydrolysis reactor feed vapor to a common distillation column. Alcohol is removed from the top of this column and returned to the esterification reactor. The liquid product from the bottom of the column comprising largely water and ester are fed to the hydrolysis reactor. The ester is continuously hydrolyzed in that reactor to form free acid. The organic acid is 70 to 85% concentration and about 1.5 moles of alcohol are used per mole of organic acid. Reaction times are 12 to 16 hours for this system. The fractionating column is located immediately on top of the two reactors. This process appears to use a relatively low level of alcohol, but this is really the initial charge to the unsteady state system. The system is only suitable for batch esterification. At the end of the batch, the ratio of methanol to lactic acid in the esterification reactor will be very high, as much as 20:1 or more. This is because during the batch the acid is gradually removed from the esterification reactor while the alcohol is continuously returned during the run. The process will be quite energy intensive as towards the middle and end of the batch there will be a need to take large amounts of methanol or alcohol into the overheads. This process is thus not suitable for large scale, continuous efficient operation. The effective average ratio of methanol to lactic acid would be near to 10:1 if this process were to be run in multiple vessels in a configuration such as might allow continuous operation. The effective vast excess of methanol is needed to obtain high yield in this equipment and process configuration.
Franke, Gabsch, and Thieme, (East German Patent 206 373, Jan. 25, 1984) describe a similar process for formation of lactic acid esters of C1, C2 and C3 alcohols with the reaction and evaporation both occurring at reduced pressure and temperature in a modified vacuum-recirculation-evaporator. They use vacuum to operate the equipment at temperatures below the boiling point of the alcohol and use high levels of sulfuric acid as the catalyst. For example, in their example 2, they charge 2.0 liter of concentrated sulfuric acid into 15 liter of 80% crude lactic acid at 50 Torr pressure. This is over 15% W/W sulfuric acid in the initial lactic acid charge. The system is heated at 60xc2x0 C. under vacuum, and then 15 to 20 liters of methanol is added slowly and continuously subsurface. As soon as the methanol contacts the lactic acid, it both reacts with the lactic acid and tends to vaporize and to carry the hot acid-alcohol-ester mixture up into the heating tube portion of the reactor and to the flash part of the reactor. The system is mixed and operates such that all three portions of the reactorxe2x80x94the heating, the evaporation, and the feed zonexe2x80x94are at essentially the same pressure, and such that the temperatures in each area are similar or perhaps the temperature in the flash area is greater than that in the reservoir due to the heat input. The liquid in the reservoir is drawn upward by convection through the heat exchanger into the vapor chamber. The liquid runs by gravity back into the reservoir. The chamber produces liquid which drains back into the reservoir and vapor. The resultant overhead condensate is 50% by weight methyl lactate. As the liquid runs back by gravity, thus the evaporation chamber and the reservoir must be at similar pressures. If the evaporation chamber was at a reduced pressure, then the liquid would not run back to the reservoir. Hence the system operates essentially identically to a single heated reactor. Other such systems can be envisaged, for example the heat could be applied in an external recirculation loop, or with internal coils, or with a jacket. In each case the vapor would be drawn off overhead as is shown in this patent.
This invention is limited to C1-C3 alcohols, low pressures, and low reaction temperatures. These low reaction temperatures require the mentioned very high levels of sulfuric acid to keep reactor volume to an economic size. However high levels of sulfuric acid, even at low temperatures, might lead to dehydration side reaction products such as dialkyl-ethers derived from the alcohols, and also various products from the degradation of lactic acid in the presence of sulfuric acid. The relative extent of these side reactions at these low temperatures is not known, but they will be minor as the yield reported in the patent is 96-98% for an extended run.
For the example of ethyl lactate, the system temperature would not exceed 78xc2x0 C. This system is quite similar to earlier references wherein the alcohol mixture is introduced subsurface into the lactic acid mixture and a mixture of water, ester and alcohol removed in the overhead vapor. In their claim 2 they note that system pressures of 15 to 50 mm Hg and temperatures of 40 to 65xc2x0 C. are favored. Their claims 3, 4, and 5 present how a modified vacuum-recirculation-evaporator may be used to advantage for operation of their process in a semi-continuous (i.e. semi-batch) mode. This involves charging the system with a batch of lactic acid, then operating the system with a continuous feed of alcohol. After a certain amount of run time, the lactic acid is depleted from the reactor device and a new batch is charged. This equipment involves a thermosiphon heat exchanger (2) that draws the liquid from the reactor into a vapor-liquid disengagement chamber (3) from which the liquid drains back into the reactor (1). A valve (9) regulates the flow through the heat exchanger. No mention is made of the need to balance temperature, pressure, concentration, holdup time, and catalyst concentration nor of the need for sufficient water to ensure successful operation.
The mole ratio of methanol to lactic acid in this process can be calculated from a typical example. A feed of 15 to 20 liter of methanol is contacted with 80% w/w lactic acid. Allowing for densities of these feeds, this represents a molar ratio of alcohol to acid of from 3.3:1.0 to 2.5:1.0. The feed mole ratio of water to lactic acid is about 1.24:1.
It is important to note that the methanol feed for the Franke process is not into the reactor but rather into the inside of the riser tube that feeds the evaporator/heat exchanger. The methanol will contact the hot lactic acid and tend to flash, carrying the hot mixture of methanol and lactic acid vapor and liquid, together with water and methyl lactate upwards through the heat exchanger to the flash chamber. Thus the reaction with the methanol occurs mainly as the methanol is added to vaporize and carry the hot boiling mixture up the reaction pipe through the heat exchanger to the flash area. Residence time and temperature in the flash unit and the reactor unit will be similar. The key difference between the Franke process and those of Filachione and Wenker is this equipment and mode of introducing the methanol such that the liquid and boiling vapors are carried upwards into the heat exchanger.
Their apparatus does not allow operation at temperatures above the normal boiling point of the alcohol, does not have any way to separate catalyst, and uses an unusually high level of catalystxe2x80x9415% w/w sulfuric acid. The overhead vapor is 50% w/w lactic acid basis in the form of methyl lactate. This is a high level of methyl lactate in the overhead vapor. The process is however limited in the scope of operation.
Datta and Tsai (U.S. Pat. No. 5,723,639) present a more modern approach that uses pervaporation or vapor permeation to achieve dehydration. A three stage system is used comprising a reactor, a water permeation system and an ester permeation system. A pervaporation membrane is used to permeate water formed in the reactor. The water is permeated into a reduced pressure vapor space. A second membrane separation system is used to selectively remove the ester formed in the reaction. As in earlier technologies, the reaction is driven by a dehydration step.
The question that is not addressed effectively in the above patents is how to drive the reaction to high levels of conversion while (1) minimizing production of undesired side products such as dimers, oligomers, and polymers, (2) minimizing energy usage, and (3) allowing continuous operation effectively with low capital cost equipment. There is a long-standing need for improved processes for producing esters of organic acids.
The current invention concerns separation of time scales. The discovery shows that under selected conditions, reaction is minimized in the equipment where mass transfer occurs, and thus formation of unwanted side reaction products such as dimers and oligomers is avoided.
One aspect of the present invention is a process for producing an ester, comprising the steps of: (a) feeding to a first vessel a feed that comprises organic acid, alcohol, and water, whereby organic acid and alcohol react to form monomeric ester and water, and whereby a first liquid effluent is produced that comprises as its components at least some ester, alcohol, and water, the components of the first liquid effluent being substantially in reaction equilibrium; and (b) feeding the first liquid effluent to a second vessel, whereby a vapor product stream and a second liquid effluent stream are produced, the vapor stream comprising ester, alcohol, and water, wherein the second vessel is maintained substantially at vapor-liquid equilibrium but not substantially at reaction equilibrium.
In one embodiment of the invention, the first vessel is operated at a pressure P1 and the second vessel is operated at a pressure P2, where P1 and P2 are substantially the same, and the average residence time of the feed in the first vessel is at least 10 times longer than the average residence time of the first liquid effluent in the second vessel.
In another embodiment, the first vessel is operated at a temperature T1 and the second vessel is operated at a second temperature T2 that is sufficiently lower than T1 so that the contents of the second vessel are not substantially close to reaction equilibrium.
In yet another embodiment, the first vessel is operated at a pressure P1 that is from about 30-500 psig and the second vessel is operated at a pressure P2 that is from about 1-14 psia.
In still another embodiment, catalyst is added to the first vessel in an amount sufficient to catalyze the formation of the ester, and where at least some of the catalyst is removed from the first liquid effluent before it enters the second vessel, so that the contents of the second vessel are not substantially close to reaction equilibrium. In certain more specific versions of this embodiment of the process, the catalyst is heterogeneous in the first vessel and is substantially not present in the second vessel. Alternatively, the catalyst is homogeneous in the first vessel and is substantially removed prior to the second vessel via washing.
Features of various embodiments of the process which can achieve the necessary separation of time scales can include:
both the temperature and pressure in the first vessel are greater than that in the second vessel;
the second vessel is operated under pressure greater than atmospheric but with short residence time (e.g., about 1 to 10 minutes);
the first vessel is operated at a liquid temperature of from 150 to 220xc2x0 C. and the second vessel is operated with an exit vapor temperature of from 30 to 100xc2x0 C.;
the first vessel is operated under pressure greater than atmospheric pressure; or
the first vessel is operated substantially in a liquid phase with pressures sufficient to substantially suppress vaporization and with temperatures up to 220xc2x0 C. without any added catalyst. In these latter systems, the organic acid (e.g., lactic acid) itself acts as the catalyst for the reaction.
In one preferred embodiment the organic acid is selected from the group consisting of mono-, di-, and tricarboxylic acids having 3-8 carbon atoms.
In another preferred embodiment the organic acid is selected from the group consisting of acetic acid, succinic acid, citric acid, malic acid, lactic acid, hydroxyacetic acid, pyruvic acid, itaconic acid, formic acid, oxalic acid, propionic acid, beta-hydroxybutyric acid, and mixtures thereof.
The alcohol preferably is an aliphatic alcohol having from 1-20 carbon atoms, most preferably an aliphatic alcohol having from 1-12 carbon atoms. Alcohols that are presently especially preferred include i-butanol, t-butanol, n-butanol, i-propanol, n-propanol, ethanol, and methanol.
The feed for the first vessel can be created by feeding a single mixed stream to the vessel. Alternatively, a plurality of streams can be fed, each containing one or more components for the feed mixture. For example, one feed stream can consist essentially of water.
Preferably the feed comprises sufficient water to substantially suppress formation of lactic acid oligomers and ethyl lactate oligomers in the first liquid effluent and the second fluid effluent. In one embodiment of the invention, the first liquid effluent is substantially liquid. Preferably the vapor product stream from the second vessel comprises at least 5% by weight each of ethyl lactate, ethanol, and water.
The second liquid effluent can optionally be recycled to the first vessel.
The feed can take a variety of forms. For example, the feed can contain lactic acid, lactic acid oligomers, and ethanol. As another example, the feed mixture can comprise one or more of polylactic acid, polylactide, poly-hydroxybutyrate, or one or more other polyesters based substantially on pure or mixed alpha or beta hydroxyacids, and can further comprise water. As yet another example, the feed can comprise more than one alcohol or organic acid, and as a result mixed esters are formed. However in this embodiment the boiling points of the alcohols, esters, and water preferably do not have a range of more than 110xc2x0 C. from the lowest to the highest boiling point.
The present invention is quite useful in conjunction with fermentation processes, so the feed can comprise crude or partially purified broth derived from fermentation of sugars that has been treated to form an stream of acidic pH. In that situation the feed mixture will typically also comprise one or more impurities selected from the group consisting of inorganic salts, protein fragments, sugar residues, ketones, and metal ions.
In one specific embodiment, the feed mixture comprises lactic acid that is substantially optically pure, that is, at least 90% optically pure and ideally 99% or greater optically pure.
Instead of using a single first vessel and a single second vessel, the process can employ a plurality of either of both, operated in series. For instance, in one embodiment, the second vessel is divided into several sub-vessels operated in series, each with temperature, pressure, catalyst, and average residence time such that they operate with vapor and liquid exit streams that are substantially not in reaction equilibrium, and such that the vapor product stream from each sub-vessel is richer in alcohol and water than the vapor product stream of the subsequent sub-vessel.
If the feed comprises polylactic acid, the feed optionally can be pretreated with hot water at temperatures of about 240xc2x0 C. and pressures of up to 500 psig prior to entering the first vessel.
It will usually be desirable to include in the process the steps of dehydrating and purifying the vapor product stream and separating from that stream ester and alcohol.
The process and equipment of the present invention are efficient for reaction systems where the feed acid or alcohol or both tend to form unwanted dimers, oligomers, and polymers when dehydrated that reduce the yield of the desired ester product. A two-stage process and equipment is used to achieve separation of time-scales for reaction equilibrium and for mass transfer equilibrium while maintaining high water levels needed to suppress unwanted oligomer formation. This invention is advantageous where the ester product is more volatile than the feed organic acid.
A suitable recycle reactor system can include two vessels, or groups of vessels. The first vessel is operated to give a product that is substantially close to reaction equilibrium. The second vessel involves vapor-liquid equilibration and phase change and produces a liquid reaction effluent that is substantially far from reaction equilibrium. The vapors removed from the second vessel or vessels include water, ester, and alcohol. A recycle stream can be passed from the second vessel group back to the first vessel, device or group.
For successful operation the conditions of temperature, pressure, holdup time, reagent concentration, and catalyst levels in the first vessel or group must be such that reaction equilibrium is approached, while the conditions in the second vessel or group must be such that reaction equilibrium in the liquid effluent is not substantially approached.
For this invention, sufficient water is required as part of the feed to the esterification reaction. If the water content of the alcohol and acid used as feedstocks to the system are too low, then water must be added. This is unusual as esterification reactions are usually driven by the removal of water. In addition to requiring water in the feed, appropriate balancing of rates of alcohol and acid feeds is necessary for best operation.
The process and equipment of the present invention can operate at high levels of water in the reaction system with good energy efficiency and good yield.
This invention is of particular value for esters made by reacting methanol, ethanol, propanol, isopropanol, isobutanol, tert-butanol, 1-butanol, or pentanols with any of the following organic acids:
aliphatic alpha hydroxy monocarboxylic acids containing from 2 to 10 carbon atoms, such as hydroxyacetic acid, lactic acid and alpha hydroxy-butyric acid;
aliphatic beta hydroxy monocarboxylic acids containing from 2 to 10 carbon atoms, such as xcex2-hydroxy propionic acid and 2-hydroxy-butyric acid;
gamma and delta hydroxyacids;
other polyfunctional compounds that include both acid and alcohol groups.
The present invention differs from prior art processes in several ways, such as the ability to use a wider range of alcohols, higher temperatures, less alcohol, and more water. The present invention makes use of the separation of vessels in an esterification system to allow independent control of the temperature, holdup time, recirculation rate, pressure, vapor fraction, catalyst level, and feed ratios. A feed dehydration step is not required in the present invention.